Electric resistance welding (ERW) refers to a group of welding processes such as spot and upset welding that produce coalescence of faying surfaces where heat to form the weld is generated by the resistance of the welding current through the workpieces. Some factors influencing heat or welding temperatures are the proportions of the workpieces, the electrode materials, electrode geometry, electrode pressing force, weld current and weld time. Small pools of molten metal are formed at the point of greatest electrical resistance (the connecting surfaces) as a high current (100-100,000 A) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials and the equipment cost can be high.
Upset welding relies on two electrodes to apply current to join metal sheets. However, instead of pointed electrodes, contacts or induction coils are used to induce current, making it possible to make long continuous welds.
Upset welding is mainly used on the seams of tubes and pipes for its ease and accuracy. The resulting weld is extremely durable due to the length of the contact area. A coil of metal sheet is uncoiled to flat, and formed continuously into a tube using forming rolls.
Tubes formed from metal sheet may be formed with a single seam or with dual seams. Dual seam mills are known where two semi-circular sections are formed separately and then brought together for welding. This process requires some twisting or deformation of the semi-circular sections prior to joining. As shown in prior art FIG. 1, welding pressure at the welding point (7) between the two semi-tubes (1, 2) is produced by deflection between the spreaders (3, 4) and the electrode rollers (5, 6). However, excessive bending or twisting will result in an inferior product, particularly as the diameter of tube becomes larger.
There is a need in the art for an improved dual seam welding process, particularly for tubular products of a larger diameter.